With the introduction of digital television and high definition video, consumer display products are being introduced to support the additional video formats and at the same time be backwards compatible with legacy video formats. Standards for digital television formats are covered by the Advanced Television Selection Committee (ASTC). The ASTC video standard includes a 1080 line/frame interlaced 60 Hz format (1080I@120 Hz), a 720 line/frame progressive format 60 Hz (720P@60 Hz) and a 1080 line/frame progressive 30 Hz format (1080P@30 Hz). The ASTC video standard also includes other formats such as 525I@60 Hz and 525P@60 Hz. A high line number indicates a high display resolution. The frequency designation is the frame display rate or the number of frames displayed in one second. A high frequency display has less large area “flicker.”
Examples of legacy formats include the National Television Selection Committee (NTSC) 525 line interlaced 60 Hz (525I@60 Hz) format and the phase alternating lines (PAL) 625 line interlaced 50 Hz (625I@50 Hz) format. In NTSC and PAL interlaced formats, there are two fields per frame, each field containing half the number of lines per frame, one field containing the odd numbered lines and the other field containing the even numbered lines. While the resolution of NTSC and PAL are low, the display equipment is relatively inexpensive, in part because low frequency equipment is less expensive to build and in part because of the maturity of the video display industry.
In displaying interlaced video, the video display unit raster displays all odd-numbered lines in one vertical sweep of the screen and then raster displays all even-numbered lines in the next sweep. With a persistent display device (i. e. the ability to maintain an image for a short time before fading) and the tendency of the human eye to average or blend subtle differences in light intensity, the human viewer sees a complete display, but the amount of information carried by the display signal and the number of lines that must be displayed per sweep are halved. In interlaced mode, the screen refresh frequency is the same as the frame display frequency. In displaying progressive mode video the video display unit raster displays each line of a frame in sequence. In progressive mode, the screen refresh frequency is the same as the frame display frequency.
Conversion between interlaced and progressive mode, between different numbers of scan lines and between different scan frequencies typically requires complex and expensive video signal conversion equipment.